DESCRIPTION

The lock functions provide synchronisation in the kernel by preventing
multiple processes from simultaneously executing critical sections of
code accessing shared data. A number of different locks are available:
struct simplelock
Provides a simple spinning mutex. A processor will busy-wait
while trying to acquire a simplelock. The simplelock operations
are implemented with machine-dependent locking primitives.
Simplelocks are usually used only by the high-level lock manager
and to protect short, critical sections of code. Simplelocks are
the only locks that can be used inside an interrupt handler. For
a simplelock to be used in an interrupt handler, care must be
taken to disable the interrupt, acquire the lock, do any pro-
cessing, release the simplelock and re-enable the interrupt.
This procedure is necessary to avoid deadlock between the inter-
rupt handler and other processes executing on the same proces-
sor.
struct lock
Provides a high-level lock supporting sleeping/spinning until
the lock can be acquired. The lock manager supplies both
exclusive-access and shared-access locks, with recursive
exclusive-access locks within a single process. It also allows
upgrading a shared-access lock to an exclusive-access lock, as
well as downgrading an exclusive-access lock to a shared-access
lock.
If the kernel option LOCKDEBUG is enabled, additional facilities are pro-
vided to record additional lock information. These facilities are provid-
ed to assist in determining deadlock occurrences.

FUNCTIONS

The functions which operate on simplelocks are:
simple_lock_init(slock)
The simplelock slock is initialised to the unlocked state. A
statically allocated simplelock also can be initialised with the
macro SIMPLELOCK_INITIALIZER. The effect is the same as the
dynamic initialisation by a call to simple_lock_init. For exam-
ple,
struct simplelock slock = SIMPLELOCK_INITIALIZER;
simple_lock(slock)
The simplelock slock is locked. If the simplelock is held then
execution will spin until the simplelock is acquired. Care must
be taken that the calling process does not already hold the sim-
plelock. In this case, the simplelock can never be acquired. If
kernel option LOCKDEBUG is enabled, a "locking against myself"
panic will occur.
simple_lock_try(slock)
Try to acquire the simplelock slock without spinning. If the
simplelock is held by another process then the return value is
0. If the simplelock was acquired successfully then the return
value is 1.
simple_unlock(slock)
The simplelock slock is unlocked. The simplelock must be locked
and the calling process must be the one that last acquired the
simplelock. If the calling process does not hold the simplelock,
the simplelock will be released but the kernel behaviour is un-
defined.
The functions which operate on locks are:
lockinit(lock, prio, wmesg, timo, flags)
The lock lock is initialised according to the parameters provid-
ed. Arguments are as follows:
lock The lock.
prio The process priority when it is woken up after sleeping
on the lock.
wmesg A sleep message used when a process goes to sleep wait-
ing for the lock, so that the exact reason it is sleep-
ing can easily be identified.
timo The maximum sleep time. Used by tsleep(9).
flags Flags to specify the lock behaviour permanently over
the lifetime of the lock. Valid lock flags are:
LK_NOWAIT
Processes should not sleep when attempting to
acquire the lock.
LK_SLEEPFAIL
Processes should sleep, then return failure
when acquiring the lock.
LK_CANRECURSE
Processes can acquire the lock recursively.
lockmgr(lock, flags, slock, p)
Set, change or release a lock according to the parameters pro-
vided. Arguments are as follows:
lock The lock.
flags Flags to specify the lock request type. In addition to
the flags specified above, the following flags are
valid:
LK_SHARED
Get one of many possible shared-access locks.
If a process holding an exclusive-access lock
requests a shared-access lock, the exclusive-
access lock is downgraded to a shared-access
lock.
LK_EXCLUSIVE
Stop further shared-access locks, when they
are cleared, grant a pending upgrade if it ex-
ists, then grant an exclusive-access lock.
Only one exclusive-access lock may exist at a
time, except that a process holding an
exclusive-access lock may get additional
exclusive-access locks if it explicitly sets
the LK_CANRECURSE flag in the lock request, or
if the LK_CANRECURSE flag was set when the
lock was initialised.
LK_UPGRADE
The process must hold a shared-access lock
that it wants to have upgraded to an
exclusive-access lock. Other processes may get
exclusive access to the protected resource
between the time that the upgrade is requested
and the time that it is granted.
LK_EXCLUPGRADE
The process must hold a shared-access lock
that it wants to have upgraded to an
exclusive-access lock. If the request
succeeds, no other processes will have ac-
quired exclusive access to the protected
resource between the time that the upgrade is
requested and the time that it is granted.
However, if another process has already re-
quested an upgrade, the request will fail.
LK_DOWNGRADE
The process must hold an exclusive-access lock
that it wants to have downgraded to a shared-
access lock. If the process holds multiple
(recursive) exclusive-access locks, they will
all be downgraded to shared-access locks.
LK_RELEASE
Release one instance of a lock.
LK_DRAIN
Wait for all activity on the lock to end, then
mark it decommissioned. This feature is used
before freeing a lock that is part of a piece
of memory that is about to be freed.
LK_REENABLE
Lock is to be re-enabled after drain. The
LK_REENABLE flag may be set only at the
release of a lock obtained by a drain.
LK_SETRECURSE
Other locks while we have it OK.
LK_RECURSEFAIL
Attempt at recursive lock fails.
LK_SPIN Lock spins instead of sleeping.
LK_INTERLOCK
Unlock the simplelock slock when the lock is
acquired.
slock Simplelock interlock. If the flag LK_INTERLOCK is set
in flags, slock is a simplelock held by the caller.
When the lock lock is acquired, the simplelock is
released. If the flag LK_INTERLOCK is not set, slock is
ignored.
p Should always point to the current process curproc.
lockstatus(lock)
Determine the status of lock lock. Returns LK_EXCLUSIVE or
LK_SHARED for exclusive-access and shared-access locks respec-
tively.
lockmgr_printinfo(lock)
Print out information about state of lock lock.

RETURN VALUES

Successfully acquired locks return 0. A failed lock attempt always re-
turns a non-zero error value. No lock is held after an error return (in
particular, a failed LK_UPGRADE or LK_FORCEUPGRADE will have released its
shared-access lock). Locks will always succeed unless one of the follow-
ing is true:
[EBUSY] LK_FORCEUPGRADE is requested and some other process has al-
ready requested a lock upgrade or LK_NOWAIT is set and a
sleep would be required.
[ENOLCK] LK_SLEEPFAIL is set and a sleep was done.
[EINTR] PCATCH is set in lock priority and a signal arrives to in-
terrupt a system call.
[ERESTART] PCATCH is set in lock priority and a signal arrives so that
the system call is restarted.
[EWOULDBLOCK]
Non-null lock timeout and timeout expires.

CODE REFERENCES

This section describes places within the OpenBSD source tree where actual
code implementing or utilising the locking framework can be found. All
pathnames are relative to /usr/src.
The locking framework itself is implemented within the file
sys/kern/kern_lock.c. Data structures and function prototypes for the
framework are located in sys/sys/lock.h.